Campaigners today accused the government of performing a U-turn over sex education in faith schools, after changes to a bill they said would allow the schools to discourage the use of contraception and teach that homosexuality is wrong.

Amendments to the children, schools and families bill allowing education to "reflect a school's religious character" had been made under pressure and would undermine efforts to reduce teenage pregnancies, they said. It meant teachers would be free to preach against sex outside marriage and not encourage safer sex.

The bill says sex and relationships education (SRE) must be taught in a way that reflects a "reasonable range" of religious, cultural and other perspectives, and promotes equality, encourages acceptance of diversity and emphasises the importance of rights and responsibilities.

But an amendment tabled by the schools secretary, Ed Balls, last week, says these rules should not be read as preventing governing bodies or heads from letting the subject be taught in a way that reflects the school's religious ethos.

On its website, the Catholic Education Service, which negotiates on behalf of bishops, claimed that the amendment had been secured by its lobbying.

Andrew Copson, the chief executive of the British Humanist Association, said the changes would deprive pupils of crucial information. "The government's amendment effectively provides an opt-out for faith schools from teaching full, comprehensive and objective SRE," he said.

The Accord Coalition, which campaigns for inclusive education, accused Balls of implicitly condoning homophobia in schools and undermining attempts to tackle homophobic bullying. Its chairman, Rabbi Dr Jonathan Romain, called on the government to withdraw the amendment, which was "made under pressure".

"It is astonishing that the government plans to deny young people of their right to accurate, balanced SRE … Children at faith schools have just as much right to information that could help them avoid an unwanted pregnancy or sexually transmitted infection."

A spokesman from the Department for Children, Schools and Families dismissed the complaints. Faith schools would not be able to opt out of statutory SRE lessons when they came into effect in September 2011, he stressed.

"All maintained schools will be required to teach full programmes of study in line with the principles outlined in the bill, including promoting equality and encouraging acceptance of diversity.

"Schools with a religious character will be free to express their faith and reflect the ethos of their school, but what they cannot do is suggest that their views are the only ones."

This meant a Catholic school would be required to teach the facts about contraception, but would also be able to reflect the church's views on its use.